Texas Instruments MSP50C6xx manual Special Filter Instructions

Special Filter Instructions

N tap filters ideally require 2N multiply–accumulates. Four instructions are provided to compute this equation: FIR, FIRK, COR and CORK. All filter instructions require overflow modes to be reset since these instructions have built in overflow hardware. In addition, these instructions must be used with a RPT instruction.

FIR and FIRK instructions perform 16-x-16 bit multiplies and 32-bit accumulation in 2 clock cycles (per tap). The FIR/FIRK instruction takes 2N clock cycles (for N taps) to execute (once inside the RPT loop). FIRK is useful for fixed filters and requires the minimum amount of data memory. However, the DP register may need to be context saved and restored since the filter coefficients are in ROM. FIR is useful for adaptive filtering or applications where coefficients are provided from an external source. FIR does not require a context save and restore for the DP register since both the buffer and the coefficients are in RAM.

COR and CORK instructions perform 16-x-16 bit multiplies and 48-bit accumulation in 3 clock cycles (per tap). Once inside the RPT loop, the total number of clock cycles for an N tap filter is 3N. The COR and CORK instructions are identical in operation and arguments to FIR and FIRK. However, an additional 16-bit extended accumulate cycle is added to prevent the arithmetic overflow common in auto correlation filters.

FIR (COR) Instructions: The execution of the filter instructions is shown in Figure 4–6. To use FIR (COR) instructions, some initial setup is required. ConsecutiveRxpair{Rxeven,Rxeven+1}shouldbechosenwithRxevenpointing

to the RAM sample buffer array and Rxeven+1 pointing to the RAM coefficient array. The MR register should be loaded with the first coefficient, h[0]. FIR

(COR) can now execute with a repeat instruction for N taps. The value of Rxeven is incremented during execution. After execution, the last value of

Rxeven points to the sample buffer location where the next sample can be stored.

FIRK (CORK) Instructions: FIRK (CORK) instructions work exactly the same was as FIR(COR) instructions, however, the coefficient array is located in program memory (ROM). Instead of loading Rxeven+1 with the pointer to coefficient array in RAM, the data pointer, DP, is loaded with the value of the coefficient array.

Circular Buffering:The easiest way to understand circular buffering is by example. Suppose a filter, h[n], has three coefficients. Then, theoretically, to calculate one output sample of the filter, the buffer should contain the current sample plus the past 2 samples. Since the output, y[k], for a three tap filter is,

y[k] = h[0]• ⋅ [k] + h[1]• ⋅ [k–1] + h[2]• ⋅ [k–2]

On the C614, the circular buffer must contain N+1 samples. In the above ex- ample, the buffer must contain four locations (which is one more location than

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